You cannot create custom date ranges using API. However, you may create one in the UI and use it in APIs to pull data for the custom date range.

There is no available interface you can get the relation IDs. You can right click > Inspect Element on the classification drop down in the Classification Importer to check the relation IDs for other classifications.

The first is a numeric identifier of the report suite. The second part is the relation ID of the classification. Since these numbers are related to the report suite and classification being used, they do impact FTP upload/classification process.

You cannot pass date range in query strings. The solution would be to use a relative date (Yesterday, Last Week, This Month etc) and create a book mark for the report. You can also use custom date ranges.

We can still do flow analysis on the values, though I admit I haven't tested it. But it will be worth giving a shot.

Segments work very differently than filters. Segments are based on containers: hit, visit and visitor.  You are using Visitor container. Assuming you are using the segment in Pages report, ALL pages from visitors who saw "campaign contains affil" (and not just the hits/Pages where the campaign was a...

You may also check the time-stamp settings of the report suite. If the report suite is time-stamp enabled, it will not show data from live hits.  

In my opinion, the closest you can get is with Merchandising Evars as they are tied to the product till purchase and can be set multiple times during a visit. 

Unfortunately, there is no way to export a Workspace project to Report Builder.

Sometimes when saving a workbook in a lower version of Excel can corrupt the file. That can also cause the Report Builder requests to disappear. You may consider recreating the requests.